The sharpening of scissors, shears, knives and similar tools is typically carried out on a device which includes a clamp or fixture for securing the tool as the tool edge is presented to the face of a grinding apparatus. These prior art devices generally fall into one of two categories. The first category consists of independent, manually held and guided clamping devices which are not affixed to the grinding wheel apparatus.
Typical of the first category of devices are those shown in U.S. Pat. Nos. 4,005,553 and 4,095,376. Both of these patents disclose somewhat similar devices which are characterized by a counterbalanced support structure which is designed to pivot downwardly toward the grinding wheel. The shear blade is clamped in the support by a clamping member which includes a trapezoidal-shaped clamping plate. The rear portion of the support is connected to a base member through an adjustment mechanism which allows the setting of the degree of pivotal movement of the counterbalanced support under the force of gravity. U.S. Pat. No. 4,005,553 shows an adjustment mechanism at the clamping plate in the form of a nut and bolt assembly while U.S. Pat. No. 4,095,376 shows a camming lever mechanism which applies the necessary clamping force on the blade to be sharpened. Both of the devices are independent, manually held operated units that the patentee states may be used with any type of grinding wheel apparatus.
The second category of devices are those devices which are integrally or structurally associated with the grinding wheel apparatus. Devices of this type are exemplified by U.S. Pat. Nos. 3,755,971 and 3,574,268. In the latter patent, the device holds a scissors so that it can be adjusted for setting the blade edge of the scissors relative to the periphery of the grinding wheel. The device also has a fixed jaw and a moveable jaw.
The above described devices have deficiencies or limitations which make them either difficult to use or limit the type of tool that can be sharpened. A common serious disadvantage of prior art devices is that these devices do not permit precise variation of the angle between the blades of the shears or scissors and the grinding wheel. Another problem in prior art sharpeners is that they generally do not accomodate various types and sizes of shears and do not apply adequate clamping force to the blade. Still another common disadvantage is that most prior art devices do not provide adequate clamping force to stabilize and secure the blade, particularly if the blade is tapered or is of irregular cross-section. Prior art clamping devices do not tolerate appreciable wear of the grinding apparatus. A principal shortcoming with such prior art devices is that they do not allow for proper clearance of the workpiece at both the clamp and the grinding wheel apparatus.
Other common disadvantages include the absence of ability to pre-set the desired sharpening angle. Further, most prior art sharpening fixtures require trial and error sharpening techniques which results in excess blade material removal and heating of the blade to the point where annealing of the heat-treated edge occurs.
As will become more apparent hereafter, prior art clamp and grinding apparatus are seriously deficient in numerous other ways.